Increase in the densities of devices on semiconductor chips has necessitated corresponding increases in circuit densities on ceramic packaging substrates used to mechanically support and electrically interconnect the semiconductor devices to input/output devices and to each other. This has led to the development and use of multilayer substrates with several layers of interconnected conductors to supply appropriate voltages to the device terminals, to communicate with other devices connected to the same substrate, and to fan out the very closely spaced terminal pads of the devices so as to make it possible to attach external connectors to them. The conductor patterns in the internal and surface planes of such multilayer substrates are commonly formed by screen printing with a paste or ink containing a substantial percentage of a refractory metal powder such as tugsten or molybdenum, the said pattern consolidating into solid metal lines upon a high temperature sintering step.
The circuit density, i.e. the number of circuit lines that could be formed in a unit area of the surface, in "thick film" circuits formed as above is severely limited by the screening techniques used to form them. Significantly higher circuit densities are realized if the circuit lines are fabricated by lithographic techniques in conjunction with vacuum deposited metal films - the so-called "thin film" techniques widely used in the fabrication of semiconductor devices. Such thin film circuit techniques are applied to the fabrication of circuit lines on ceramic and glass-ceramic substrates to decrease the interconnection lengths and to decrease the number of buried layers needed in multichip, multilayer substrates.
Thin film technology has made great strides in the fabrication of semiconductor devices to the point that patterns consisting of micron wide lines spaced a micron apart are routinely mass produced today. While in the case of semiconductor device fabrication, defects in isolated portions of the silicon wafer can be simply dealt with by discarding one or more chips located in the defective area, defects on any part of a large, multichip substrate surface could lead to the rejection of the entire substrate because of the extensive interconnection between all areas of the substrate. Thus, a major challenge facing the use of thin film technology on large, multichip ceramic substrates is the ability to fabricate defect-free thin film conductor patterns over a large area or, the ability to easily repair defects such as shorts and opens. Based on extensive experience from semiconductor fabrication technology, it is estimated that the ability to repair just a few such defects can make the difference between zero yields and yields in the 90% range.
Methods for repairing shorts, i.e. unwanted metal between conductor lines, are straightforward. These methods include laser ablation, abrasive jet trimming, selective etching and use of a mechanical cutting means to cut away the bridging metal. When the bridging metal is non-adhering to the surface underneath, an ultrasonic horn can be used to effectively remove the bridging metal as described in U.S. Pat. No. 4,504,322. In this method the region of the bridging or shorting metal is subjected to the action of intense ultrasonic vibrations produced in a coupling liquid medium such as water until the non-adhering metal spalls off. The opens that form in thin film metallization line patterns are most frequently caused by fine dust particles or fibers that adhere to the substrate surface. While the larger dust particles are normally removed by conventional cleaning methods used in thin film processing, the fine particles cause opens typically of a micrometer in length or less. It is impractical to fabricate large, multichip substrates with thin film conductor patterns in the absence of simple and effective repair techniques for open defects.
There are, however, a paucity of methods for localized repair of open defects. One method for repair of substrates having such open defects is to completely remove the defective pattern by etching or grinding and to repeat the lithographic and metallization steps. This strategy is expensive and, moreover, does not guarantee freedom from similar defects in the reworked pattern. In addition, there are many technical barriers to overcome to accurately overlay a repair pattern over the original pattern.
Since the open defects occur randomly across the thin film pattern and many different circuit patterns are utilized in substrates, and it is desirable to avoid accurate alignment requirements between the repair metal and substrate pattern, it is desired that the repair method should be non-specific to the pattern of the thin film lines at the location of the defect, but rather that it be universal. The use of a pre-patterned repair film should be precluded since it would require a new decal design each time a different circuit pattern was used or an open occurred in a different location, which would be very often. This would be extremely impractical and time consuming to attempt to implement. If the repair process requires accurate alignment of the repair metal with the open defects, the process would once again become impractical and uneconomical. Also, in the interest of not creating new defects in the originally defect-free areas of the thin film pattern during the repair process, it is desirable to limit the deposition of the repair metal substantially to the local area of the open defect, and not over a majority of the substrate. Hence, the repair method should be universal in nature and localized in size.
U.S. Pat. No. 4,259,367 discloses a method of repairing opens and shorts in semiconductor packages. The '367 patent demonstrates the complex steps required to repair thin film lines in the existing art. Utilizing the method of the '367 patent one must first use a laser beam or electron-beam to cut conductors on either side of shorts to convert them into opens, then depositing an insulating layer, and etching vias in the insulating layer to by-pass shorts and bridge opens in the underlying metallurgy. Then, strips of metallization are deposited to connect the respective vias in the insulation layer. This method is very complex and requires precise alignment and processing for many steps.
It is also undesirable to have to implement masks or the like to define where repairs should be made. Since the opens occur randomly, a unique mask, each covering a different area of the substrate, would be required for almost every repair situation. This would be very costly and extremely impractical to implement. Moreover, overlaying a mask on the delicate thin film metallization patterns is likely to cause further damage to the pattern. A universal, maskless repair means that can be implemented at any time in the substrate manufacturing process is desired.
U.S. Pat. No. 4,442,137 discloses a maskless method for overlaying protective metal coatings on a prior metal pattern on ceramic substrates. In this method, the metal chosen for overcoating is blanket deposited by vacuum evaporation or sputtering over the total substrate surface, covering both the metal pattern on it as well as the bare ceramic areas in between. The blanket deposited metal exhibits little discrimination in its physical adhesion to either the ceramic surface or the surface of the underlying pattern. Thus, the metal is bonded to both the pre-existing pattern and ceramic substrate. This is mostly due to the fact that atomic contact is made between the blanket deposited metal and the ceramic substrate because the inherent roughness of the ceramic substrate causes the substrate surface to trap sputtered or evaporated metallization which is deposited. The substrate is then heat treated in a suitable ambient to a temperature at which sufficient diffusion bonding occurs between the deposited metal and the metal pattern underneath while, simultaneously, promoting debonding and delamination of the deposited metal on the bare ceramic areas. A higher temperature than required to merely diffusion bond is required to debond the deposited metal from the ceramic. The debonding forces are created by the thermal expansion mismatch between the deposited metal and the ceramic substrate. Once the relative bonding and debonding is achieved, the substrate surface is then subjected to the action of an ultrasonic horn in water to cause the selective removal of the debonded metal from the bare ceramic areas leaving only the prior circuit pattern coated with the deposited protective metal.
The method of U.S. Pat. No. 4,442,137, however, is not well suited for repairing open defects by forming a metal bridge across the defects. In the '137 method, the metal film is directly deposited on to the substrate surface by vacuum deposition techniques. In the direct metal deposition, the metal is deposited everywhere on the substrate surface because deposition only onto repair areas through a mask is impractical as the occurrence of open defects is random and differs from substrate to substrate and may damage other metallization on the substrate. The direct deposition of metal onto the circuit pattern will result not only in the build up of metal thickness of the circuit lines but also in the increase in the width of the circuit lines and, hence, in the decrease in the spacing between the lines. This also increases the likelihood of causing shorts between the lines. In addition, since the heat treatment must delaminate the metal film from the ceramic surface in addition to promoting the diffusion bonding of the overlay metal to the circuit lines, the temperatures required to accomplish the process are relatively high in comparison to the temperatures required for diffusion bonding. Higher temperatures may cause excess deformation of the metal patterns and also tend to degrade the adhesion of the patterned thin film metal lines to the ceramic surfaces. Moreover, when using gold as the repair metal, (which is a preferred repair metal because), direct deposition entails additional expense for the gold deposited on areas not required to have the reapair.
Thus, the 4,442,137 method is not suited for or intended for repairing thin film lines in the first place, and if it were applied to a repair operation, there would be numerous technical difficulties that would make such an application undesirable, and practically unfeasible.
In view of the above there is a need in the art for an improved method of repairing randomly occurring opens in conductive thin film circuit line patterns. There is also a need in the art for the repair technique to require no precise alignment of the repair metal over the defect area, no masks, and to require no photolighographic processes. It is also desired not to have costly sputtering or evaporation steps. There is also a need to have the repair film selectively bond to the patterned metallization, but not bond to the ceramic substrate so that high temperatures for debonding the excess metal would not be required, and there would be little chance of having residual metal cause shorts or increased line widths.
There is also a need for the repair means to be universal so that any random opens can be repaired, regardless of the exact shape of the open, or where it occurs on the pattern. The deposition of the repair metal should also be limited to the local area of the open.